Stone Mastic Asphalt (SMA), an example of a thin surface dressing, was developed in the early 1970s in Germany (originally to resist the wear of studded tyres) where to date, over 250 million square meters of highway have been paved with this product. The successful production of SMA is typically made possible with the use of stabilising additives in the form of highly specialised cellulose fibres. These prevent excessive drainage of the asphalt binding agent (bitumen).
The advantages of thin surface dressings over conventional road surface applications are now well recognised and its use is increasing at a steady pace. For example, due to its excellent characteristics and performance, SMA has been adopted in many countries, including the Netherlands, France, Switzerland, UK, Norway, Finland, Sweden, Denmark, Turkey, Greece, Poland, Japan, Israel and the USA.
Asphalt production plants which manufacture blends of aggregate and bitumen for use in the production of thin surface dressings, were not designed with these new products in mind. Such plants consist of large storage vessels holding bulk raw materials, such as aggregate and bitumen, which are then conveyed to a central elevated mixing box for blending. The asphalt mixing box typically has a height similar to that of a three or four story building. The raw materials are typically mixed in a ratio of 1 tonne of aggregate to 200 kg of fillers and binders, the latter typically comprising 70-100 kg of bitumen. Blending takes place at an elevated temperature of typically 170° C. However, the industry is moving towards production of technically more sophisticated thin surface dressings that require the incorporation of multiple additives. In contrast to the relatively large proportion of bitumen in a finished asphalt product, these further additives are incorporated in relatively small amounts, such as 3 kg of fibre, 10 kg of pigment and 5 kg of a polymer modifier per tonne of aggregate.
Accordingly, these new products are manufactured today either by manual addition of the multiple additives into the asphalt mixing box or by means of a supply system as shown in FIG. 1.
The additive supply system shown in FIG. 1 comprises components and operates as follows. A bulk silo 10 for storing an additive in pelletised form is filled by means of a filler pipe 12. Additive can be added to the bulk silo 10 by being blown up filler pipe 12. A safety ladder 14 enclosed by safety rails 16 provides access to a hinged roof hatch 18 at the top of bulk silo 10. A high level probe 20 located at the top of bulk silo 10 indicates when the silo is full. Gas is exhausted down exhaust pipe 22, to which a cyclone dust collector 24 is fitted. The bulk silo 10 is emptied by agitation of its contents at low level using rotary electric vibrators 26. A level probe 28 and an emergency low level probe 30 respectively indicate when the bulk silo 10 is nearly or completely empty. Additive leaving the silo is conveyed by a supply auger 32 to the asphalt mixing box. The supply auger may be provided with a calibration or sampling point 34 and an acoustic flow detection sensor 36. The auger 32 is driven by a drive unit 38 which draws additive to outlet 40 and into the asphalt mixing box. The elevated asphalt mixing box is accordingly fed with additive from the bulk silo 10 from above. The quantity of additive added to each batch of asphalt is controlled simply by the run duration time of drive unit 38.